Disc drill bits are one type of drill bit used in earth drilling applications, particularly in petroleum or mining operations. In such operations, the cost of drilling is significantly affected by the rate the disc drill bit penetrates the various types of subterranean formations. That rate is referred to as rate of penetration (“ROP”), and is typically measured in feet or inches per hour. As a result, there is a continual effort to optimize the design of disc drill bits to more rapidly drill specific formations and reduce these drilling costs.
Disc drill bits are characterized by having disc-shaped cutter heads rotatably mounted on journals of a bit body. Each disc has an arrangement of cutting elements attached to the outer profile of the disc. Disc drill bits can have three discs, two discs, or even one disc. An example of a three disc drill bit 101, shown in FIG. 1A, is disclosed in U.S. Pat. No. 5,064,007 issued to Kaalstad (“the '007 Patent”), and. incorporated herein by reference in its entirety. Disc drill bit 101 includes a bit body 103 and three discs 105 rotatably mounted on journals (not shown) of bit body 103. Discs 105 are positioned to drill a generally circular borehole 151 in the earth formation being penetrated. Inserts 107 are arranged on the outside radius of discs 105 such that inserts 107 are the main elements cutting borehole 151. Furthermore, disc drill bit 101 includes a threaded pin member 109 to connect with a threaded box member 111. This connection enables disc drill bit 101 to be threadably attached to a drill string 113.
In this patent, inserts 107 on discs 105 are conically shaped and used to primarily generate failures by crushing the earth formation to cut out wellbore 151. During drilling, a force (referred to as weight on bit (“WOB”)) is applied to disc drill bit 101 to push it into the earth formation. The WOB is translated through inserts 107 to generate compressive failures in the earth formation. In addition, as drill string 113 is rotated in one direction, as indicated by arrow 131, bit body 103 rotates in the same direction 133 as drill string 113, which causes discs 105 to rotate in an opposite direction 135.
Referring now to FIG. 1B, another type of disc drill bit, as disclosed in U.S. Pat. No. 5,147,000 also issued to Kaalstad (“the '000 Patent”) incorporated herein by reference in its entirety, is shown. The '000 Patent discloses a similar three disc drill bit to that of the '007 Patent, but instead shows another arrangement of the inserts on the discs of the disc drill bit. In FIG. 1B, inserts 123 are disposed on the face of discs 125, instead of on the outside radius. The primary function of inserts 123 is to cut out the borehole by generating compressive failures from WOB. After inserts 123 generate the primary compressive failures, they then perform a secondary function of excavating the compressively failed earth. The conical shape and location of inserts 123 on disc drill bit 121 are effective for generating compressive failures, but are inadequate in shape and location to excavate the earth formation also. When used to excavate the earth formation from the compressive failures, inserts 123 wear and delaminate very quickly.
Although disc bits have been used successfully in the prior art, further improvements in the drilling performance may be obtained by improved cutting configurations.